Light emitting diodes (LED) have recently been used for illuminating devices as LED light bulbs in place of conventional incandescent light bulbs because of their lower energy consumption. LED light bulbs are required to have a light distribution property similar to that of conventional incandescent light bulbs.
However, blue LED chips have an emitted-light distribution property of close to Lambertian, and therefore a white LED or a bulb-color LED light source using blue LED chips with fluorescent substances also have a light distribution property close to Lambertian. Lambertian means a distribution of emitted light in which intensity of the emitted light is proportional to the cosine of the observation angle. Accordingly, with light sources using LEDs, radiation to areas at the rear of the light source, that is, radiation with a solid angle of 2π or more cannot be realized, and intensity of light toward the front of the light emitting surface becomes strong. Under the situation, methods for radiating sufficient amount of light omnidirectionally with solid angles of 2π or more can be roughly divided into two groups. The first is a method in which LED chips are arranged three-dimensionally in each direction. However, in this method manufacturing cost is high and there exist problems concerning thermal design of LEDs. The second is a method an optical element is arranged in front of the light source to control directions of rays. However, there has not existed an optical element by which an ideal light distribution can be efficiently realized.
On the other hand, illuminating devices in which paths of rays emitted from the light source are changed by an optical element have conventionally been developed. Such conventional illuminating devices are those using a point light (as shown in Patent Documents 1 and 2, for example) or those using a surface light source and an optical element which is large enough with respect to a size of the light source and which is used to illuminate light in a certain direction by setting the focal length to a value as large as possible for higher degree of collimation (as shown in Patent Document 3, for example).
Further, in general, control of directions of rays for other purposes than collimation (for example, for dispersion of rays) becomes easier as a size of the optical element with respect to that of the light source increases. The reason is that as a distance of an optical surface of the optical element from the light source increases, incident directions of rays to various positions on the optical surface become more uniform, and therefore the rays can be controlled more freely. In contrast, when a size of the optical element is similar to that of the light source, an optical surface of the optical element is close to the light source. As a result, incident directions of rays to various positions on the optical surface are not uniform, and therefore the rays can hardly be controlled freely.
Thus, a compact illuminating device in which an optical element is used to distribute light emitted by a surface light source such as an LED to illuminate sufficient amount of light omnidirectionally with solid angles of 2π or more has not been developed.